Free tool
A refractometer is brilliant before fermentation, but once there's alcohol in the sample it reads wrong. Enter your original and current Brix to get corrected gravity and an ABV estimate.
Most refractometers need a correction factor around 1.04 for wort, so check yours against a hydrometer if you can. FG and ABV are good estimates, not exact.
A refractometer measures how much light bends through your sample, which tracks sugar content (in Brix) very accurately, until fermentation starts. Alcohol bends light too, so a reading taken during or after fermentation overstates the remaining sugar.
By comparing your original Brix with the current Brix, this calculator estimates how much alcohol is present and corrects for it, giving you a usable final gravity and ABV from two quick refractometer readings.
Refractometers are calibrated for sugar in water. Once alcohol is present (during and after fermentation) it bends light differently, so the raw Brix reading is no longer accurate. This calculator uses your original and current Brix to correct for the alcohol and estimate true gravity and ABV.
Refractometers read slightly high on wort because wort is not pure sucrose. Dividing the reading by a correction factor (typically around 1.04) brings it in line. The exact figure varies by instrument, so calibrate yours against a hydrometer if you can.
The correction (Sean Terrill's cubic model) is a well regarded estimate and good enough for most homebrewing, but it is not as exact as a hydrometer final gravity reading. Treat the FG and ABV as close approximations.
For your original gravity, yes, refractometers are quick and need only a couple of drops. For final gravity you need this correction (or a hydrometer) because of the alcohol. Many brewers take OG with a refractometer and confirm FG with a hydrometer.
Demijohn Journal logs your gravity readings over a batch and works out ABV as you go. Free, in your browser.
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